Planetary imagers are useful for remote sensing of atmospheric compositions, crop assessments, weather prediction and other types of monitoring activities. Monochromatic and multispectral satellite-based, remote sensors are able to measure properties of the atmosphere above the earth, when their detector arrays are properly calibrated for radiometric response.
A method of calibrating the radiance measured by these remote sensors is to create a reference radiation using a known source of spectral irradiance, such as the sun. The radiation from the sun may be used as a reference signal which, in turn, may provide a known radiance to a remote sensor for calibrating its detector arrays.
The output of the detector arrays may be measured as the remote sensor receives the known energy from sunlight or a diffusive reflector. This radiance calibration method provides sufficient information to correctly measure and calculate other types of radiance incident on the remote sensor during normal operation, when using the output of the remote sensor, as the remote sensor views the earth or other target of interest.
In the prior art, there are at least four different methods for calibrating the radiance measured by the sensors. One of these methods is the use of Calibration Light Source Assemblies (hereinafter “CLSA”). CLSAs use the sun as a source of illumination and provide a partial aperture illumination. A second method is the use of a Full Aperture Calibration Door (hereinafter “FACD”). An FACD provides a coating on the inner surface of a calibration door. During the calibration process, the door is opened and the coated inner surface reflects the sun towards the aperture. This provides full or partial aperture illumination. A third method is called Full-Aperture Calibration Surface (hereinafter “FACS”). In the FACS method, a medium or coating is applied to a surface. The surface is then moved into position to reflect the sun as a source of illumination. As with the FACD method, this method provides full or partial aperture illumination. Finally, another method that may be employed is an On-board Calibration Source (hereinafter “OBCS”). In the OBCS method, incandescent lamps, light emitting diodes, or other portable electromagnetic sources (including, but not limited to, radiative black bodies) are used to provide the illumination. The lamps, diodes, and/or other sources are positioned in front of the aperture or otherwise placed such that they illuminate direct energy towards the sensor when calibration is required.